I would be more than a little skeptical - as in I wouldn't believe it for a minute - about rhododendron roots 3 feet down no matter what kind of soil its growing in.

As to the main question, my experience in growing rhododendrons among tree roots - some of them from Norway maples, even - is that rhododendrons will grow fine if perhaps a little more slowly as long as they receive enough water. You have to remember that not only will the trees compete for water at the root level, but also their canopies will shed rain just like an umbrella.

Planting trees and rhododendrons at the same time should work even better as the tree roots and those of the rhododendron will start out on an equal footing and will adjust to coexistence. Shallow fibrous trees like Japaese maples will be especially compatible. In George Schenk's book, The Complete Shade Gardener, theres an entire very valuable chapter about the best and worst trees for shade gardening.

As for planting under established trees, 6 feet is more than you need, imho. Forget about digging. Use logs, stone, etc. to create a raised bed on a portion the soil surface. (If you're using loose, organic rhododendron-friendly soil, it's not going to harm the tree in the slightest.) The rhododendron root mass will, as long as it doesn't suffer drought, compete successfully with the tree.

Thanks mainegrower. I'll be on the lookout for a copy of that book...I'm near a couple great horticultural libraries.

If you are growing a rhody near a tree, you can root prune the tree by sticking a shovel in around the drip line of the rhody. This cuts roots of the tree that my be a problem and make the rhody form a denser root structure inside the drip line.

Some trees that are more compatible with rhodies are:

Ginkgo biloba, Maindenhair Tree
Metasequoia glyptostroboides, Dawn Redwood
Nyssa sylvatica, Sour Gum
Quercus rubra, Red Oak

Here is a link that might be useful: Companion trees for rhododendrons and azaleas

Cutting roots has exactly the same effect as pruning branches: it stimulates rapid regrowth in an even denser manner than existed before. Far better, in my view, to plant in a raised area and avoid the stimulative effect of root cutting.

My experience with root pruning is where I am growing rhododendrons near walnut trees. I find root pruning very effective. I can keep the walnut roots out of the area and the rhododendron roots within the area. I use a spade and a mattock and cut everything down as far as they will reach.

You are right, root pruning stimulates root growth, but mostly in the area where the remaining roots are growing and fine roots remain, not in the area that was isolated by the root pruning. Of course, this is not a one time thing, but neither is a raised bed. Roots will come into any area within the drip zone and beyond. Root pruning plus installation of a root barrier will keep the roots out much longer. But those that do come in will be tunneling under the barrier and will be harder to prune unless you prune at a different point away from the barrier.